Eisenmenger's syndrome (or ES, Eisenmenger's reaction or tardive cyanosis) is defined as the process in which a left to right shunt caused by a congenital heart defect in the fetal heart causes increased flow through the pulmonary vasculature, causing pulmonary hypertension,[1][2] which in turn causes increased pressures in the right side of the heart and reversal of the shunt into a right-to-left shunt.

Eisenmenger's syndrome can cause serious complications in pregnancy,[3] though successful delivery has been reported.[4] Maternal mortality ranges from 30% to 60%, and may be attributed to fainting spells, thromboembolism, hypovolemia, hemoptysis or preeclampsia. Most deaths occur either during or within the first weeks after delivery.[5] Pregnant women with Eisenmenger syndrome should be hospitalized after the 20th week of pregnancy - or earlier if clinical deterioration occurs.

The larger and more muscular left side of the heart generates the high pressure required to supply blood to the brain and body. The systemic circulation derived from the left heart is a high pressure circuit. The smaller, right side of the heart generates the lower pressure required to circulate blood to the lungs. The pulmonary circulation derived from the right heart is a low pressure circuit.

If a significant anatomic defect (i.e. a hole or breach) exists between the two sides of the heart causing a shunt, blood will flow from the left side to the right side (because the right side has less resistance) in a curve proportional to the magnitude of the breach. This results in higher than normal blood flow and pressure directed to the lungs. High blood pressure in the right heart circuit (from any cause) is generally known as pulmonary hypertension. A sustained intracardiac breach or shunt from the left to the right circulation then results in maladaptive remodeling of the left and right heart ventricles to accommodate the mismatched pressures encountered. The beat-to-beat volume of blood pumped through a left-to-right breach is a percentage of cardiac output (CO). Clinically a low index or percentage of CO ejected through a shunt is harmless; a high index or percentage of CO ejected through a left-to-right shunt heralds Eisenmenger's physiology.

Increased right-sided blood pressure causes a cascade of pathologic damage to the delicate pulmonary capillaries, causing them to be incrementally replaced with scar tissue. Scar (dead lung tissue) does not contribute to oxygen transfer, therefore decreasing the useful volume of the pulmonary vasculature. The scar tissue also provides less flexibility and compliance than normal lung tissue, causing further increases in pulmonary blood pressure, and the weakened heart must pump harder to continue supplying the lungs, leading to damage of more capillaries.

Eventually, due to increased resistance and decreased compliance of the pulmonary vessels, elevated pulmonary pressures cause the myocardium representing the right heart to hypertrophy, commonly understood as right ventricular hypertrophy (RVH). Eisenmenger's syndrome-derived RVH may cause an increase in right heart pressure that becomes sufficient to reverse blood flow through the breach, allowing blood to travel from the right side of the heart to the left side. The body is then supplied with deoxygenated blood (which bypassed the lungs by flowing directly from the right heart to the left heart), leading to cyanosis and resultant organ damage. In an infant with significant Eisenmenger physiology the skin is typically insufficiently oxygenated resulting in the antiquated medical term of "blue baby syndrome".

The reduction in oxygen transfer reduces oxygen saturation in the blood, leading to increased production of red blood cells in an attempt to increase oxygen delivery. The excess of red blood cells is often labeled polycythemia although only one blood cell line is increased. Desperate for enough circulating oxygen, the body begins to tax the bone marrow (where red blood cells are generated) and dump immature red cells into the spleen and bloodstream. Immature red cells are not as efficient at carrying oxygen as mature red cells and they are less flexible and less able to easily squeeze through tiny capillaries in the lungs. This contributes to the death of pulmonary capillary beds. The increase in red blood cells also causes the hyperviscosity syndrome.

A person with Eisenmenger's syndrome is paradoxically subject to the possibility of both uncontrolled bleeding due to damaged capillaries and high pressure, and random clots due to hyperviscosity and stasis of blood.

If the hole in the heart is identified before it causes pulmonary hypertension, it can normally be repaired through surgery preventing the disease.[10] After pulmonary hypertension, a heart–lung transplant or a lung transplant with repair of the heart is a possible option. Transplantation is the final therapeutic option and only for patients with poor prognosis and quality of life. Timing and appropriateness of transplantation remain difficult decisions.[5] 5-year and 10-year survival ranges between 70% and 80%, 50% and 70%, 30% and 50%, respectively.[11][12][13] Since the average life expectancy of patients after lung transplantation is as low as 30% at 5 years, patients with reasonable functional status related to Eisenmenger syndrome have improved survival with conservative medical care compared with transplantation.[citation needed]

Various medicines and therapies for pulmonary hypertension are under investigation for treatment of the symptoms.[14]